1. Field of the Invention
The present application relates to an optical module that outputs a polarization combined beam, in particular, the application relates to an optical module providing a polarization dependent optical isolator but outputs a polarization combined optical beam.
2. Background Arts
When an optical module outputs an optical signal that includes two optical beams each having a polarization perpendicular to the other, an optical isolator with of a polarization independent type is implemented to suppress backward optical beam. Light emitted from a laser diode (LD) and returning the LD reflected at a Fresnel interface of an end surface of an optical fiber and so on, the quality of the laser optical beam considerably degrades. An optical isolator put in the optical path from the LD effectively prevents the optical beam from returning to the LD. Two types of the optical isolator have been known, that is, the polarization dependent isolator (PDI) and the polarization independent isolator (PII). The former isolator substantially cuts the backward optical beam by a combination of two polarizers and a Faraday rotator, while, the latter isolator reduces the backward retuning to the LD by deflecting the optical axis of the backward optical beam.
Some optical modules includes a plural LDs arranged in on a line and outputs an optical beam combined with respective optical beams output from the LDs. In such an arrangement, the backward beam deflected by the PII possibly enters to neighbor LDs although one PIT may be placed for the combined beam. A PDI may substantially cut the backward beam; but, the PDI is unable to set for the polarization combined beam. It would be a best solution to prepare a plurality of PDIs each corresponding to respective LDs, but this arrangement requires an enough space to install PDIs within the transmitter optical module.